I would asap go and get some B complex Liquid you can get it from TSC in the cattle dept it will say injectable but can be given orally 1 ml per duckling a day or go to Walmart etc and get human equivalent in the vitamin section it will be 3ml a day since it isn't as potent as TSC Make sure she can dunk her head and clean out her eyes. But not get in water and not be able to get out. But the leg problems are most likely Niacin and the Nutritional yeast is great for ducklings but sometimes there will be one that just needs something extra and all the Bs seem to be doing the job this season in getting them back up and going. Please keep us updated.
Most that have been using the Liquid B complex have been mixing it into some thawed peas smashed or Meal worms just enough treats so they get the full amount of the vitamins.